This invention relates to electrophotographic systems utilizing jump monocomponent development.
In electrophotographic imaging systems an electrostatic charge image is formed on the surface of a photoconductive insulating member which is conductive when exposed to light and insulating in the dark by first applying an electrostatic charge uniformly to the surface to the layer in the dark and then exposing the layer to a light image. Thereafter, the electrostatic charge image is made visible by applying toner which adheres to the charge image portions but not to the uncharged portions of the surface and the resulting toner image is then transferred to a substrate such as paper.
In two-component development systems, insulating toner particles having an electrostatic charge are carried by carrier particles to the surface containing the charge image and the toner particles are deposited on the image from the carrier particles which are then removed.
In another development process, called monocomponent development, there are no carrier particles and the toner particles are deposited on the surface containing the charge image from a developer roller which passes adjacent to the surface containing the charge image. In one form of monocomponent development the toner particles are retained on the development roller by electrostatic adhesion and the development roller is spaced by a gap from the surface containing the charge image so that the toner particles must jump from the development roller to the image-bearing surface. In order to overcome the adhesion of the toner particles to the developer roller and facilitate jumping of the toner particles to the adjacent electrostatic charge image, an electric potential is applied between the developer roller and the image-bearing layer. The magnitude of that potential depends on the width of the gap between the image-bearing surface and the surface of the development roller and must be high enough to cause the toner particle to jump to the electrostatic image portions of the surface but not to the non-image portions of the surface. If the potential difference between the developer roller surface and the image-bearing surface is too high, however, the air in the gap between the developer roller and the image-bearing surface may become ionized and cause arcing between the developer roller and the image-bearing surface.
The voltage difference between the threshold voltage which is the voltage that is just sufficient to cause the toner particles to jump to the image-bearing surface and the maximum permissible voltage which is the voltage that will not produce arcing or cause particles to jump to non-image areas is called the voltage width, and the electrostatic image potential minus the background potential on the photoreceptor is called the contrast potential. One approach to assuring effective jump development without causing arcing or deposition in non-image areas is to increase the gap between the developer roller and the image-bearing surface until the voltage width approaches but does not exceed the contrast potential. This approach, however, does not tolerate significant variations in contrast potential or in the size of the development gap. Another possible approach is to reduce the charge on the toner particles and maximize the size of the toner particles so as to minimize adhesion forces between the toner particles and the developer roller but this increases the possibility that toner particles might be transferred to nonimage portions of the photoreceptor surface.
U.S. Pat. No. 4,629,669 discloses jump monocomponent development using magnetic toner particles held on the developer roller by magnets within the roller.
U.S. Pat. No. 5,737,671 discloses an electrophotographic photoreceptor which includes a transparent substrate, a transparent conductive layer coated on the transparent substrate, and a thin film intermediate layer made of semiconductor material or semiconductive insulating material having a band gap of 2.4 eV or larger. The thin film intermediate layer is applied by a vacuum deposition method and layered on the transparent conductive layer, and an amorphous silicon photoconductive layer is layered on the thin film intermediate layer. This electrophotographic photoreceptor is used in an image forming method which includes an exposure/developing step for carrying image exposure with an exposure device located on the transparent substrate side of the photoreceptor and, at substantially the same time, carrying out image development with a bias voltage applied to the photoreceptor by a developing device provided on the other side of the electrophotographic photoreceptor.
U.S. Pat. No. 5,824,445 discloses a process for producing an image which includes the steps of electrically charging a photoreceptor in the dark, forming a latent image on the photoreceptor by selective imagewise exposure to light, and developing the latent image on the photoreceptor with a two-component developer using a magnetic carrier using which requires a photoreceptor with a conductive substrate and a photoconductive layer of amorphous silicon with a thickness of 25 .mu.m or less.
U.S. Pat. Nos. 5,660,960 and 5,824,444 disclose an image forming apparatus containing a photoreceptor which includes an endless transparent support layer coated with a transparent conductive layer, a charge carrier generation layer and a charge carrier transport layer in which the thickness of the charge transport layer is selected to provide desired charge mobility.
Schein, Electrophotography and Development Physics, Rev. 2d Ed., p. 137 (1996), discusses the relations between photoreceptor thickness and developer roller voltage in jump development, showing that the developed mass per unit area can be represented by the equation: EQU M/A=V.epsilon..sub.0 /[Q/M(d.sub.s /K.sub.s +.LAMBDA./.nu.)],
where:
M/A=Developed mass per unit area PA1 V=Applied Voltage PA1 .epsilon..sub.0 =Permitivity Constant PA1 Q=Charge PA1 d.sub.s =OPC Thickness PA1 K.sub.s =Dielectric Constant of OPC PA1 .LAMBDA.=L/K.sub.E (Development Gap divided by Effective Dielectric Constant) PA1 .nu.=Speed Ratio